The invention relates to fabricating parts out of ceramic matrix composite material (CMC material).
CMC materials are generally used for parts that need to present good mechanical behavior up to high temperatures, typically 1200° C. and even above, in an oxidizing environment.
Applications for CMC materials are to be found in the aviation and space fields, for example for structural parts that are exposed to a stream of hot gas in an aero-engine.
CMC materials are formed on a fiber reinforcing substrate that is densified with a ceramic matrix. The substrate may be made of carbon fibers or of ceramic fibers, such as refractory oxide or nitride or carbide fibers (typically silicon carbide SiC).
A method of making CMC material is described in document U.S. Pat. No. 4,752,503. In that known method, an interphase coating is formed on the fibers so as to optimize bonding between the fibers and the matrix, i.e. so as to have bonding that is strong enough to transfer to the fiber reinforcement the mechanical stresses to which the material is subjected, but bonding that is not too strong so as to avoid making the material fragile, strong bonding encouraging cracks to propagate from the ceramic matrix and through the fibers, thereby degrading the fiber reinforcement. The interphase is typically pyrolytic carbon (PyC) or boron nitride (BN). The interphase can thus be formed by a succession of individual layers of PyC (or BN) and of SiC, thus contributing to deflecting cracks, as described in an article by R. Naslain et al., published in the Journal of Solid State Chemistry, Academic Press USA, Vol. 117, No. 2 (2004-2), pp. 449-456.
A method commonly used for densifying the fiber reinforcement substrate of a CMC material is chemical vapor infiltration (CVI). A reaction gas is introduced into an oven in which the temperature and pressure conditions are suitable for encouraging the gas to diffuse into the pores of the fiber reinforcement and to from the matrix by depositing on the reinforcing fibers a material that is produced by decomposing one of the constituents of the reaction gas or by a reaction taking place between a plurality of constituents thereof.
Another known process for densifying a porous substrate by a ceramic matrix is densification by a liquid technique. The substrate is impregnated by a composition in the liquid state that contains a precursor for the ceramic material of the matrix, e.g. a precursor in the form of a resin. The precursor is transformed by heat treatment to produce the ceramic material of the matrix. Such a liquid process implemented on a fiber texture having fibers coated in a boron nitride interphase is described in a document EP 0 549 224.
The fiber reinforcement substrate is made in the form of a preform of shape that corresponds to the shape of the part that is to be made. The fiber preform is obtained from fiber texture(s) such as in particular unidirectional textures, yarns, tows, woven fabric, or two-dimensional textures, one dimensional or multi-directional sheets or felts by methods such as winding, two- or three-dimensional weaving, braiding, draping (superposing plies of two-dimensional textures on a former), superposing plies of two-dimensional texture and bonding them together by needling, stitching, etc.
In order to conserve the shape desired for the fiber preform during densification, in particular when the part to be made is complex in shape, it is necessary to have recourse to support tooling. Such tooling occupies a large amount of space and represents a large amount of thermal inertia in a CVI oven. Thus, the densification of a preform with a ceramic matrix as obtained by CVI is performed in two steps. A consolidation first step is performed during which a ceramic matrix consolidating phase is deposited so as to bond together the fibers of the preform sufficiently strongly to enable the preform to conserve its shape without the help of tooling. After consolidation, the preform is withdrawn from the tooling and densification is continued during a second step.
Nevertheless, the CVI process is slow and consolidating preforms by such a process occupies a considerable length of time, with support tooling present in the oven, thereby leading to the above-mentioned drawbacks (occupying space and constituting thermal inertia). Furthermore, after consolidation, the preforms need to be cooled down, extracted from the oven so as to withdraw the support tooling, and then reinserted into the oven, and raised again to the desired temperature in order to continue densification, thus implying a large amount of manipulation.
Proposals have been made in an article by A. Ortona et al., published in Fusion Engineering and Design, Elsevier Science Publishers, Amsterdam, Netherlands, Vol. 51-52 (2000), pp. 159-163, to make a composite material part of the SiC—SiC type (fiber reinforcement and matrix both made of SiC) by a method comprising:                using a CVI process to form a carbon interphase on the SiC fibers of a fiber preform held in tooling;        then using a CVI process to form a first consolidating matrix phase of SiC within the fiber preform while still supported in tooling; and        using a liquid process (polymer infiltration and pyrolysis) to form an SiC matrix phase finishing off the densification of the preform.        
Document US 2003/0162647 discloses a method comprising forming a fiber preform out of SiC fibers and performing heat treatment. After a carbon interphase has been formed by CVI, a first matrix phase is made out of SiC by CVI, followed by a second matrix phase made out of SiC by a liquid technique, with a final deposit of SiC by CVI so as to close the cracks in the second matrix phase and form an SiC coating.
A similar method with a carbon interphase being formed by CVI, an SiC first matrix phase by CVI, an SiC second matrix phase by a liquid technique, and an SiC deposit by CVI for sealing the composite material is described in EP 1 277 716.
The above-mentioned documents have recourse to a CVI process for forming an SiC first matrix phase after an interphase coating has been formed on the fibers, with the above-mentioned drawbacks of CVI processes.